Silver soldering union nipples
Chris aids newcomers to the hobby with a technique he evolved for an essential task.
Many readers will regard the process I describe here as elementary, but beginners to model engineering won’t necessarily know how to silver solder union nipples onto small diameter pipes. It took me a while to realise how useful 0.75mm silver solder wire is. When using the normal 1.5mm section silver solder, I’ve often needed to file surplus solder off the nipple before the union nut will seat.
The method described gives reliable, neat results.
Materials
You will need;
1) Pipe to which nipple will be soldered
2) Union nut
3) Nipple
Items 1, 2 and 3 must be carefully matched. The nipple must be a neat fit on the pipe, and the union nut must slip over the shank of the nipple but not the shoulder – carry out a trial assembly before committing materials.
4) Silver solder wire. 0.75mm dia
5) Flux powder
6) Citric acid solution or vinegar. (Citric acid is available from home-brew shops)
Tools
1) Wire wool
2) Spent match
3) Round-nose pliers
4) Side cutters
5) Pilot light for gas torch
6) 3.5 kw gas torch
7) Robust pliers
8) Small file
9) Centre drill
10) Vice or hearth for holding pipe during the heating process
11) Individual jam jar or similar small glass vessel
12) Protective equipment: gloves, goggles, fire extinguisher.
Procedure
1) Prepare the pipe end; file the end of the pipe so that it is square to the axis. Remove any burrs from the outside diameter with a file, and any burrs from the bore with a hand-held centre drill. Blow through to clear any swarf.
2) Clean the pipe end and the silver solder with wire wool.
Note: If this is the second nipple to be soldered onto the same pipe, make sure that both union nuts are on the pipe, and facing in the correct direction, that is, threaded ends facing outwards towards the ends of the pipe.
3) Make a ring of silver-solder wire using the round nose pliers. Make the ring a single complete turn and a firm grip on the pipe. When satisfied with the grip, cut the ring from the wire stock with side cutters and push it onto the pipe to be soldered, very near to the end.
4) Put a little flux powder into the glass jar. 5) Very little water is required to make a flux paste. Using the spent match, transfer a single drop of water to the flux powder, and mix it to a smooth thick paste.
6) Again using the spent match, spread flux paste onto the pipe close to the solder ring. 7) Push the nipple onto the pipe, as far as it will go, sliding the solder ring and flux away from the end of the pipe, so that the solder ring is in contact with the nipple and largely covered with flux paste.
As far as possible, avoid smearing any flux onto the outer diameter of the nipple. Anywhere there is flux, the solder may go, and it is preferable to avoid the need to file off surplus solder later.
8) Support the assembly to be soldered in a vice or soldering hearth, so the pipe slopes slightly downwards away from the nipple, so that the nipple will not fall off during heating.
9) Light the pilot light and from it, light the 3½ kw torch. Turn off the pilot light. Start heating the joint, holding the torch at least 20cm away from the joint to be soldered, directing the heat to the pipe at least 5cm from the solder. Should the heat be applied immediately to the area of the joint, the flux will blow away.
10) The water holding the paste will evaporate, leaving the flux as a coherent powder. From this point, the heat can be applied progressively to the joint.
11) Continue to apply heat to the joint. As it approaches red heat, the flux will become a clear liquid. Shortly afterwards the solder itself will start to soften and run. Continue applying heat to the joint until the solder forms a smooth shiny fillet.
Avoid overheating the nipple itself, or there is a risk it will become plastic and degrade its profile.
12) Turn off the gas torch and put it down where the hot nozzle cannot burn anything, especially the gas supply line.
13) Using robust pliers, drop the assembly into a dilute citric acid solution. If it is still hot, most of the glass-like flux will crack and fall off. Beware; it will spit acid.
Around a quarter of an hour in citric acid solution or vinegar will remove the rest of the flux.
14) Remove the assembly from the acid and wash it in tap water. Make sure that the pipe is not blocked with solder, by running some water through the pipe.
15) Examine the quality of the fillet. If incomplete, add more flux paste only and reheat it. Dress off any minor discolouration with the judicious use of wire wool, avoiding damage to the sealing areas of the nipple.
16) Dry the pipe and fit it. With careful direction of the flame, the pipe can be annealed again, without damage to the solder fillet.
17) Important: Clean and dry the area, and put all the tools and materials back into storage. Ensure that any soft solder and silver solder are kept completely separate from each other.